Functional determinants of forest recruitment over broad scales

AimThe drivers of tree recruitment over large spatial scales remain unexplored. Here, we ask whether species potential for recruitment and the strength of density-dependent processes, both inferred from species relative abundances, show emerging patterns that can be explained upon the basis of information about climate and functional traits. LocationEastern forests of the USA. MethodsWe document the geographical distributions and magnitudes of seedling recruitment and the strength of density dependence and conspecific density dependence for the forests of the eastern USA spanning >1.2 million km(2) across 88,854 local communities comprising 164 tree species. We also compiled climatic variables and 16 traits representing several important ecological axes of tree functional strategies to assess which factors were most strongly associated with the emerging broad-scale spatial patterns. ResultsStrong geographical variation in the potential for seedling recruitment and a latitudinal change from negative to positive density dependence moving northward were associated with adaptation to seasonal freezing temperatures and seed size. Wood density and leaf nitrogen, in contrast, were related to the magnitude of the negative density dependence and conspecific density dependence, respectively, which were prevalent over most of the region. Main conclusionsOur results provide strong evidence that tree recruitment and the strength of density-dependent processes have broad-scale patterns that can be explained by a few key species functional traits.